1. Field of the Invention
The present invention relates to a solid golf ball, and more particularly, to a solid golf ball excellent in homogeneity, rebound performance and durability, and a method of manufacturing the same.
2. Description of the Background Art
Conventionally, the core of the solid golf ball has been manufactured by heating a rubber composition having a co-curing agent such as metal salt of unsaturated carboxylic acid and a free radial initiator blended into a rubber component mainly composed of polybutadiene, to crosslink main chains of rubber molecules. For the metal salt of unsaturated carboxylic acid above, zinc acrylate is generally used to be graft-polymerized onto the main chains of polybutadiene by the free radical initiator such as dicumyl peroxide to form co-crosslinks. The basic physical properties of rubber composition after crosslinking, and properties of a golf ball formed thereof are greatly affected by the state in which the metal salt of unsaturated carboxylic acid or the like is dispersed in the rubber composition, the rate at which the metal salt of unsaturated carboxylic acid or the like reacts to crosslink to the main chains of rubber molecules by the free radical initiator, or the crosslinking formation between the metal salt of unsaturated carboxylic acid or the like and the rubber molecules.
Particularly, the temperature at which an organic peroxide serving as the free radical initiator is decomposed or the blended amount thereof considerably affects the basic physical properties of the core such as coefficient of restitution, hardness or the like. In U.S. Pat. No. 4,546,980, for example, for blending of the core using metal salt of unsaturated carboxylic acid as the co-curing agent, a technique to blend at least two types of free radical initiators (organic peroxides) having different half-lives has been proposed to improve coefficient of restitution. In the technique, at least two types of organic peroxides are dispersed and blended in a substantially uniform manner from a center portion to a surface portion of the core. In crosslinking, however, a temperature gradient is caused from the center portion to the surface portion of the core, and thus the surface portion has a high crosslink density and becomes hardened, which results in poor durability.
Another technique to apply a coat of higher fatty acid or metal salt of higher fatty acid on a particle surface of zinc acrylate used as the co-curing agent to improve dispersibility of the zinc acrylate in the rubber composition has also been proposed (U.S. Pat. No. 4,561,657 and Japanese Patent Laying-Open No. 60-92781). Still another technique to use, as the co-curing agent, metal salt of unsaturated carboxylic acid having an average particle diameter of not more than 5 μm, or unsaturated carboxylate having a particle size distribution of 0.1–5 μm and an average particle diameter of 1–4.5 μm to improve dispersibility of the metal salt of unsaturated carboxylic acid in the rubber composition has also been proposed (Japanese Patent Laying-Open Nos. 8-196661, 9-235413, 11-57068 and U.S. Pat. No. 6,136,906).
These techniques are preferable in that the dispersibility of the co-curing agent in the rubber composition is improved and hardness of the rubber composition is increased. However, due to the co-curing agent so finely dispersed, crosslink density between the main chains of rubber molecules, which is most likely to contribute to rebound performance, is decreased, whereas the pattern of graft polymerization of the co-curing agent with the main chains of rubber molecules, which is unlikely to contribute to rebound performance, is increased, which results in unsatisfactory rebound performance.
As such, the inventor of the present, invention has already proposed a method to use a microcapsule, which is the co-curing agent coated with a thermoplastic resin, to adjust the rate of crosslinking between the main chains of rubber molecules and the rate of graft polymerization between the main chains of rubber molecules and the co-curing agent so as to improve rebound performance (Japanese Patent Laying-Open Nos. 2002-85591 and 2002-159595).